Social Genetic Effects for Litter Size of Sows Housed in Groups during Gestation

Reproductive data (N=7152 sows with 8787 records) were used to estimate breeding values for total litter size under a model fitting both additive and social genetic effects. Pregnancy tested sows which failed to farrow (<1% of records) were allocated trait values of 0. Sows were penned into 1504 groups of between 2 to 10 sows/group during gestation. Seven subsets of data containing a single record per sow were created for re-estimating additive and social breeding values. Variance ratios for social genetic effects (s²) and non-genetic group effects (g²) were very low (s² ≤ 0.009 and g² ≤ 0.04) compared to corresponding estimates of heritability (h² ≤ 0.21). Correlations between breeding values across subsets averaged 0.19 and 0.09 for additive and social genetic effects. Therefore, social genetic effects for total litter size were lowly reproducible for sows across different groups. Breeding values from models fitting social genetic effects better described the mean phenotypic performance of groups than did breeding values from additive models without social genetic effects.